Redux is a popular JavaScript library used for managing the state of an application in a predictable and centralized manner. It's commonly used with frameworks like React to handle the state of a user interface. Redux follows a unidirectional data flow pattern and emphasizes the use of a single immutable state tree. Actions are dispatched to describe changes, and reducers handle these actions to update the state. This helps in creating maintainable and testable code by separating concerns and providing a clear way to manage application state.